Evaluation and modeling of the wastewater treatment plants (WWTP) have to be based on thefact that the pollutant concentrations and flow rates change constantly. In addition, different approaches mustused due to there are different processes in a WWTP. A Mexican petrochemical complex has a WWTP thatprocesses wastewaters from several petrochemical plants, where the flow rates and pollutant concentrationschange constantly. The actual WWTP has an equalization pond (EP) followed by an aerated lagoon (AL). TheEP performance was evaluated by CFD tool and it was possible to evaluate the equalization pond performance.In the biological process, a more realistic dynamical model to evaluate the AL performance was developed andcalibrated. The reactor was modeled as a plug flow reactor with high dispersion, due to there is no biologicalsludge recirculation. It was found that with an increase of 12.5% in the wastewater flow rate the Mexicanregulation will not be satisfied The model improves modeling because considers different actual operationalfactors as lost of VOC and variations of temperature, influent COD and flow rate, which have tremendousimpact on the petrochemical wastewater treatment plant performance.